Fluidized-bed reactors generally consist of the following elements: an inlet plenum for the fluidizing gases; a distributor device for the fluidizing gases, such as an orifice plate, a bubble-cap plate, or horizontal orifice pipes; the fluidized bed region where a dense bed of particles mixes with bubbles of reactant gases, and where heterogenous catalytic reactions, heterogenous gas-solid reactions, or physical transformations of the solid or gases may occur (e.g., adsorption, drying or particle growth); and a freeboard region where particles splashed from the fluidized bed have an opportunity to decelerate and drop back into the fluidized bed region before they are entrained from the reactor with the exhaust gases. There may also be a low-efficiency particulate control device such as a cyclone that will remove the majority of particles from the exhaust gas and either dispose of the collected particles or recycle them to the fluidized bed region, and possibly a high-efficiency particulate control device such as a barrier filter that will protect downstream equipment from contamination or erosion due to entrained particles, or will protect the environment.
In most fluidized-bed reactor systems the volume of the system will be dominated by the freeboard region (having a volume of about 1 to 10 times the fluidized bed volume) and by the particulate control equipment. There is little incentive to attempt to minimize the volume of the fluidized bed itself since the cost of the reactor system will be set by the freeboard and particulate control equipment. In some cases the freeboard region may be the site of significant chemical conversion itself, but in most fluidized-bed reactor applications the freeboard region is simply a large volume that does not contribute to the chemical or physical transformations of interest. Likewise, the particulate control equipment will occupy a large volume that does not contribute to the key transformations. Thus, the problem that is evident is the waste of equipment volume and its resulting high cost.
High-efficiency particulate control devices of the barrier filter class generally form filter cakes at the boundary that must be removed by either mechanical or hydrodynamic forces. Typically, filters are periodically back-blown by a high-pressure stream of clean gas to break off the filter cake and reduce the pressure drop across the filter. The filter then operates in an unsteady, cyclic fashion of slowly increasing pressure drop as the filter cake thickness increases, followed by a sudden pressure drop reduction as the cake is removed. The cost and complexity associated with the cleaning of the filter is significant. This identified another problem area characteristic of these systems.
U.S. Pat. No. 2,548,875 to Degnen et al., dated Apr. 17, 1951, describes a method for contacting gases and solids. Gases are passed through a contact chamber containing a catalyst where porous filter elements are used to separate the catalyst particles from the gas. The filters are totally or largely submerged in the dense phase of catalyst in the fluidized bed of the contact chamber and impingement of circulating catalyst particles on the surface of the filter is said to keep the surface of the filters relatively free of adherent catalyst particles to prevent an undesired increase in pressure required to force the gas through the filter.
As described in said patent, a portion of the gas, containing catalyst, may be removed from the vessel and separately treated to remove solids so as to maintain a sufficiently high velocity of gas through the vessel, which gas may be passed to a cyclone separator to remove solids therefrom and then returned to the vessel with further incoming gas to the vessel.
An object of the present invention is to combine a fluidized bed region with a filtering system in a single vessel so as to eliminate or reduce the freeboard region above the fluidized bed, eliminate the need for intermediate solids separation equipment such as cyclones, and provide for continuous self-cleaning of the bulk of the filter units used in the separation of particulates from a hot gaseous stream.